Make sure you have connected your game to Game Center. It is the only way to save, recover and sync your progress.
1. Go to the Settings view of your device
2. Find and select Game Center
3. Tap on your Apple ID
4. If you already have an account, please log in. If not, create an Apple ID for yourself.
5. Go back to your Home Screen and launch the game. You should see the Game Center notification banner on top of the screen with a message "Welcome back" including your Game Center profile name!